Heat Transfer Flashcards
(5 cards)
what is the prandtl No and what does it describe? What is the importance of its value.
Do you know the relation between heat transfer rate to wall shear stress?
IN CONTEXT WITH BOUNDARY LAYERS - laminar region over flat plate.
prandtl No. is cp x u / k (see formula in Heat transfer 2)
It describes the relative contribution of diffusion of momentum and thermal energy. a value of Pr = 1 indicates similarity between velocity and temperature profiles within the thermal and fluid mechanic boundary layer thickness.
Relation between heat transfer rate and wall shear stress, Plz see heat transfer hand out 2
Describe the difference between shear stresses for thermal and fluid mechanics for laminar and turbulent flow
Turbulent flow has an extra turbulence viscosity which is much higher than the kinematic viscosity. Understandably, because of the turbulence and the extra mixing of heat, the shear stresses are higher.
Describe the turbulent prandtl number. Why is it like that?
Prandtl number is very close to 1 in turbulent flow. Because as flow flows, it mixes and diffuses heat very well. Momentum diffusivity is very close to thermal diffusivity.
What is the reynold’s analogy for turbulent flow and how is it different to laminar flow? What is it useful for.
what’s the difference in this expression between turbulent and laminar flow.
It describes the relation between heat flux(q-dot) and shear tress under turbulent conditions. It’s very useful because to determine heat flux, it’s easier to find wall shear stress and hence find heat flux instead of convective heat transfer coefficient h.
The only difference between turbulent and laminar is the prandtl number. In turbulent flow, due to better mixing and approximations involved, the prandtl number is just 1 and hence neglected.
What is the nusselt numberl?
It is the ratio of the heat transfer of convection/ heat transfer due to conduction. OR how much more heat transfer you would get due to convection as compared to just conduction.
